More than 70 million people in North America are nearsighted.Myopia is the medical term for nearsightedness. Myopia occurswhen an eye is too long for the cornea’s curvature. Light raysentering the eye do not come to a sharp focus on the retina atthe back of the eye. Instead, they focus further forward,producing a blurred image. This is demonstrated in thediagram above. 

The term ‘nearsighted’ means that you cansee ‘near’ objects clearly without yourglasses, but objects further in the distanceare blurry. There are varying degrees ofmyopia or nearsightedness. When the eye is myopic or nearsighted, it is possible to place a lens in front of the eye to bring the image into clear focus on the retina. The moremyopic you are, the more blurred objectsappear in the distance, the higher your eyeglass prescriptionand the thicker your glasses. 
When glasses are added…

Of all myopic people, about 90% have corrections less than-6.00 diopters. Click Here for a table that shows the categories of severityfor myopia: